There is the expression, "See Something - Say Something".

The Chattanooga shooter supposedly had depression but still had guns and went to shooting ranges. No one said anything.

He got the guns from somewhere yet undisclosed. His parents didn't know their depressed son had guns?

He wrote a cryptic text to a friend and the friend doesn't question it?

I applaud the Boston cop who turned in his son for terrorist plotting.

It would be a very difficult task but would you call the FBI on your child if you had a very strong suspicion they might be plotting terrirism?